I'm riding a Kickr Snap trainer with Zwift, and I'm curious how it measures my cadence. I don't have a sensor on for that, but it seems to know not only when I'm pedaling, but measures mu cadence very closely. How is it doing this?

The Wahoo Kickr Snap comes with a power meter and speed sensor, but no cadence sensor. You'll need a standalone cadence sensor (Ant+ and USB adapter with PC or Bluetooth with phone/tablet).

I understand that, but it knows when I'm pedaling and can count cadence pretty closely. On Zwift, when I stop pedaling my avatar does. How is Zwift counting cadence through the Snap?

variation in power, I think. I always thought that should be able to work, but never looked at the data. Most apps don't do that, I have a wahoo cadence device

A standalone cadence sensor like this would work:
https://a.co/iRgCbXr

I don't know for certain but I can take an educated guess. The Snap measures power likely by sensing the resistance (physical, not electrical) against the roller. This resistance is not constant: it will be highest when you are on a downstroke and lowest when you are at the bottom of a stroke. So, measure the instantaneous resistance and count peaks per unit of time, then divide by 2. Voilą, you have cadence.

Good "guess!" Sounds very reasonable to me. Maybe I'll add a cadence counter and heart monitor, as well. I think heart rate might be good to know.